Solar Energy: Your Questions and Answers


SolarUK: Glossary

Heat store

Stores the suns energy as heat captured by the collector.

A heat store works in a similar way to the calorifier except the water in it merely stores the sun’s energy as heat captured by the collector: This stored energy is transferred to the water which comes out of the taps via another heat exchange element/coil. Heat stores can have a range of heat input sources including conventional gas, oil and solid fuel boilers. Also, ground source and solar thermal energy can be stored. Each input needs to have a separate heating coil within the store so that the different systems can maintain hydraulic separation; alternatively, heat stores can be directly heated if the fluid contained within them is circulated out of the vessel to the heat source or external heat exchanger and then returned to a different port by a pump or thermosyphon (the movement of particles in liquid or gas through natural convection). In this respect heat stores are very similar to storage cylinders except that a ‘heat store’ stores heat, which is then drawn from the storage medium by heat exchange, whereas stored hot water is stored in cylinders/clarifiers, heated there, and flows out from the cylinder to taps and showers.

Solar storage tanks can be configured in many ways and one advantage of a store over a cylinder is that hydraulic separation is achieved between the volume of the vessel and the water which will be delivered from your taps: this reduces the perceived risks associated with stored hot water at low temperatures due to the volume in the extract coil being considerably less than the cylinder volume. A disadvantage is that for it to work effectively, the store temperature needs to be much higher than the equivalent cylinder volume. The efficiency of the solar thermal system is also improved when it is heating a cylinder as there are less stages of heat transfer and the lower part of the cylinder is replenished with cooler water as draw-off occurs. Some systems will incorporate two-tanks, with the solar energy heating a heat store. The extract coil in the store would pre-heat the domestic hot water before it enters a conventional water heater/calorifier where its temperature is at the desired level before it is delivered to the outlets. This type of system is favoured in large buildings where a recirculation loop is needed and significant risk from legionella is present due to varied and inconstant water demand and the old existing infrastructure.

More information can be found here: Cylinders, calorifiers and heat exchangers

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Other Associated glossary terms : Commercial

Solar Thermal
The collection of the sun's energy as heat rather than converting it to electricity.

Vacuum Tube Solar Collector
This is a technology that uses a glass tube, think of a long thermos flask to insulate the solar collector and ensure that effectively all solar energy is retained.

Feed in Tariff
A method to reward the producer of renewable energy (electricity) by paying a fixed amount of money for each unit of electricity generated.

Recirculation - pumped secondary
Pipework carrying secondary hot water.

Sequential heating arrangement
The heating that occurs as the solar thermal system heats the water before the conventional heat source provides any necessary additional heating.

Heat store
Stores the suns energy as heat captured by the collector.

Refers to any period when the heat transfer fluid is not circulating, even though the solar panel is receiving the suns energy.

Primary and secondary systems
This refers to the water travelling round the boiler and the water in the cylinder which is heated by it.

Stratification / de-stratification
The layering of hot water on colder water.

Expansion vessel
A trapped volume of pressurised air or gas inside a steel vessel to allow for the expansion that accompanies temperature increases.

Buffer vessel
A tank storing heated water in anticipation of variable demand requirements.

Pre-feed cylinder
A cylinder placed between the cold water feed and the existing hot water cylinder.

Any pipe with several outlets to or from other pipes, or a junction of pipes.

A water storage vessel, which has the capability to increase the temperature of the water contained within it.

The controller compares the different temperatures at specific locations in the solar systems circuit.

Circulation module / Pump module
Operates the heat transfer system.

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